The companies that produce DNA (like GenScript) can make a strand of arbitrary length using any bases. This includes the normal A T C and G, but could also include methylated version as needed.
Testing for methyl groups has been around for a while by using methyl-sensitive DNA cutting enzymes. This procedure would just help you determine which sites to add methyl groups when you are manufacturing your fake DNA. Anyone who has access to a lab (or company) that can make DNA can probably also test for methylation.
One just has to be a bit more careful when producing DNA for crime scenes, though the free market of underground DNA forgers should come in line with the technology soon enough.